The energetic efficiency of the small hermetic compressors of refrigeration is greatly attributed to the good performance of its valves in the control of the gas flow.
Refrigeration compressors for domestic use employ one-way valves that control the gas flow during operation. A suction valve controls the gas flow, which comes from the suction line connected to the low pressure side of the refrigeration system, and which is drawn through the compression cylinder, while a discharge valve controls the gas already compressed, to be directed to the high pressure side of the refrigeration system.
The suction and discharge valves usually comprise one or more orifices for gas passage, and vanes affixed by one of the ends thereof, so that upon occurring a pressure differential through the valve, the vane is displaced, allowing the gas to pass in the preferred required direction.
In order to facilitate the manufacturing process, said suction and discharge orifices usually have a circular cross section and are constructed in a steel plate known as the valve plate. In most cases and mainly when the hermetic compressor has a small size, the geometrical shape of the suction valve and of the suction and discharge orifices, as well as the requirement of insulating the already compressed gas from the gas that is at suction pressure (such requirement being mainly fulfilled by the cylinder cover), forces the discharge orifice to be positioned eccentrically in relation to the axis of the center of the compression cylinder, being very close to the wall of said cylinder (FIG. 1), so that the suction orifice may also be contained within the axial projection of the internal contour of the compression cylinder, maintaining a certain minimum spacing in relation to the discharge orifice.
However, during discharge, it is necessary to supply the gas with an additional power, in order to overcome the energy losses in the valve and in the discharge muffler before flowing to the refrigeration system. This additional power may be called over-pressure power and decreases the energetic efficiency of the compressor. In this situation, the piston is very close to the upper dead point of the mechanism, resulting in a compression chamber with small height, which greatly increases the load loss of the gas that has to flow to the discharge orifice and then flow through the discharge valve. The more eccentric the position of the discharge orifice, the greater will be the loss. An example of this occurrence is described in Brazilian patent application PI 6,793,538, where such effects are discussed in relation to irregular loads imparted to the vanes, affecting the reliability thereof. However, in this prior solution, since the compressor is not small, the discharge orifice may be positioned in the center of the compression chamber.
In the construction of large and medium size compressors, in the region of the cylinder cover, there is enough space to provide a change in the direction of the gas flowing from the suction pipe to the suction orifice, by means of a curve with a radius and a cross section, which are required for allowing the whole cross sectional area of the suction orifice to be fully and homogenously used by the gas flow coming from the suction pipe. The curvature imparted to the gas flow upon passing from the suction pipe to the suction orifice, or orifices, may be achieved so that the whole cross section of the suction orifice is adequately used for gas flow passage.
Besides the situations in which there is availability of space, as mentioned above, it should be noted that in the solutions such as that described in PI 6,793,538, a suction chamber is provided in the cylinder head, directly opened to the suction orifice, eliminating the problem of a sudden change in the direction of the gas flowing from the suction pipe directly to the suction orifice or orifices.
During compression, the suction valve is submitted to a load which is calculated by (Pcil−Ps)Ao, where Pcil is the pressure in the cylinder, Ps the evaporation pressure and Ao is the area of the orifice. In the know solution of vane constructions for a suction valve, the vane bends over the suction orifice and is submitted to a bending stress, which, when superior to a limit fatigue stress of the vane material, makes the valve break due to bending fatigue. The stress over the vane is a function of the shape of the orifice. The circular orifice results in higher stress over the valve, exactly in the central point, because this point is equidistant from the seating regions.